Heat generators using combustible fuels such as oil, coal, gas or refuse materials and the like, generate a substantial quantity of waste materials in the form of pollutant gases and particulates. Federal and state environmental requirements have imposed maximum emission standards for these waste materials. Compliance with these emission standards involves substantial investments for appropriate pollution control equipment, the costs for which can be prohibitively high.
For example, large systems are available to remove particulates using a dry flue gas treatment. Typical devices used for this purpose may involve electrostatic precipitators, bag houses and the like. These devices are suitable for the removal of the particulates, but gaseous pollutants are not removed and as can be appreciated, the addition of these devices increases cost and reduces the efficiency of the heat generator.
The magnitude of gaseous pollutants generated from the combustion of fuel throughout the world is enormous. As a result, many techniques have been described for the removal of these pollutants from flue gases exhausted from heat generators. A general statement of various wet scrubbing processes for pollutant removal from flue gases exhausted from large scale electric power plants can be found in a chapter entitled "Wet Scrubbing Process--SO.sub.x and NO.sub.x Removal Chemistry" by R. G. Nevill, at page 9-312 of "Energy Technology Handbook" edited by D. M. Considine and published by McGraw-Hill Book Company.
Flue gas wet scrubbing techniques also involve substantial investments with complex systems. For example, in the U.S. Pat. Nos. 3,320,906 to Domahidy and 3,733,777 to Huntington, wet scrubbers are described in which flue gases are passed through a filter bed for intimate contact with a wash liquid. The wash liquid may be an aqueous bisulfite salt solution such as described in the Huntington patent or such alkaline scrubbing liquors indicated as useful with the wet scrubber described in U.S. Pat. No. 4,049,399 to Teller.
Since corrosive liquid droplets are likely to be entrained by the scrubbed flue gas, special techniques such as described by Teller or in the U.S. Pat. No. to Brandt 3,844,740 may be used to avoid corrosion on subsequent equipment such as an induced draft fan located at the stack where the flue gas is exhausted to atmosphere.
Another technique for the removal of pollutants may involve cooling of the flue gas to such low temperatures that gaseous pollutants such as SO.sub.2 and SO.sub.3 condense out. One such system is described in the U.S. Pat. No. to Maniya 3,839,948, in which the flue gas is cooled to about 10.degree. C. to condense that the sulfurous pollutants after which the flue gas is reheated before discharge to atmosphere.
These and other techniques for the removal of waste materials from flue gas involve a substantial amount of energy, much of which is irretrievably lost. As a result, the overall efficiency, i.e. the energy available for sale from a power plant is significantly reduced.
Techniques for preheating of air have been known and used for many years in connection with boilers to improve combustion. One such preheating technique employs a Ljungstrom air preheater. This uses a rotor through which on one side flue gas is passed while an inflow of combustion air is passed through the other side, with the two gas flows being in opposite direction. Air preheaters, however, are operated at sufficiently high temperatures to avoid condensation inside the heat exchanger of pollutants such as SO.sub.3 present in the flue gas.